Guppy Study Reveals Costly Nature Of Having Big Brains

As the most intelligent animals on the planet, we often wonder–how did we get so darn smart?

While there are many hotly contested theories, a new study from researchers at Uppsala University in Sweden supports the notion that large, energy-expensive brains evolved in primates at the cost of other bodily functions, also known as the Expensive Tissue Theory.

If bigger and better brains were always selected for, then every animal would have them, scientists rationalized. Therefore, there must be an obstacle to this evolution. The Expensive Tissue Hypothesis suggests that the energy demand of a big brain places an evolutionary limit on its size. For example, human brains take up only 2 percent of the body, yet they consume 20 percent of our energy requirements.

The hypothesis goes further by suggesting that our digestive capabilities evolutionarily suffered, but that loss was made up for by an intelligence that enabled more efficient hunting and gathering.

Because of a lack of experimental data, the Swedish researchers designed an experiment to test the Expensive Tissue Hypothesis, which was formulated based on species comparisons and correlations.

“We provide the first experimental evidence that evolving a larger brain really is costly in terms of both gut investment and, more importantly, reproductive output,” said Niclas Kolm, an ecologist from Uppsala who co-authored a report on the study, which appeared in the latest edition of the journal Current Biology.

In the study, the team artificially selected guppies in an attempt to divide them into two camps: those with larger brains and those with smaller brains. They were successfully able to breed “smarter” guppies with brains measuring about 9 percent larger than their counterparts.

The two groups of guppies were then put through a series of numerical learning tests that were designed to test cognitive ability. They found that female guppies with bigger brains were significantly better at the task–their male counterparts saw no benefit from a larger brain.

The researchers also noted a difference in digestive capacity, or gut size. The average size of larger-brained male guppies´ gut was 20 percent smaller and the females saw an 8 percent drop in gut sizes.

“Our demonstration of a reduction in gut size and offspring number in the experimental populations selected for larger relative brain size provides compelling experimental evidence for the cost of increased brain size,” the report said.

The Swedish fish researchers also found a connection between brain size and reproductive capacity. On average, the bigger brained fish produced 19 percent fewer offspring in their first clutch. However, the scientists noted that selective forces in the wild might aid the long-term reproductive capacity of smarter guppies.

“Because cognitive abilities are important to facilitate behaviors such as finding food, avoiding predation, and obtaining a mate, individuals with increased cognitive abilities are likely to have higher reproductive success in the wild,” the authors added, in their report.

The study, while thorough, left several questions unanswered–including why the females´ bigger brains correlated into higher test scores. The scientists theorized that their tests may have had a sexual bias toward females, whose foraging abilities depend on cognitive capacity.